1. Field of the Invention
This invention relates to an audio signal processor circuit for treating the signals from a telephone subscriber's line when these have been received at the telephone exchange, and converted to sampled digital form.
2. Description of Related Art
In a telephone exchange, the signals on the telephone subscriber's line usually in analogue form and are converted from analogue to digital form by a sampling and coding operation performed by an audio processor circuit. The impedances which are presented by each individual telephone line at any particular time are likely to give an impedance mismatch and this can cause a 2-to-4 wire hybrid echo signal to appear. It is possible for at least part of this echo effect to be cancelled digitally in the audio processor unit. The echo cancellation operation may be static, that is, its operation will be pre-set by the manufacturer of the circuit at some compromise impedance matching value. Alternatively, the echo cancellation could be designed to work by a process of continuous adaption to minimise the echo signal.
If sinusoidal signals happen to be present on the telephone line instead of speech, any adaptive operation can lead to the occurrence of further unwanted side effects in the adaption circuit output.
It is possible to overcome some of these problems by providing an adaptive echo cancellation system which can be modified in response to the statistical properties of the signal input, but a conventional adaptive system using a conventional signal correlator would need a substantial quantity of hardware for its realisation. The correlator hardware and necessary signal processing would be likely to increase significantly the area of circuit board necessary for that subscriber at the exchange. This could lead to a substantial increase in the cost of building the exchange.